My blocks often occur because I want the song to go in one direction, and it's headed in another. If I keep trying to make the lines fit my ideas, I can get hung up. Often, what works is not to just scrap lines, but come up with whole new ideas and quit trying to force something to work. The "wait a while" approach works, because your attachment to what the song should be has a chance to fall out of your head. Sometimes you do find a way to express exactly what you want though. In that case, it may be you have some idea of lines you can't get to work right, and the wait makes your committment to them fall out of your head.

This waiting isn't necessary if you can give up what you've already got and try something completely different. You can save your old ideas, but try something brand new just for the heck of it.

And I say this while I'm currently stuck on the last two lines of a song! I think I already wrote them - they're the second-to-last two lines. Maybe I just need a couple of almost-last lines.